The boys Angela kills while camping threw sand and laughed at her after she was thrown in the lake by Judy and Meg. Is that worthy of murder? No but neither is the general bullying that takes place in this film. It's an unfair trade off of her "eye for an eye" M.O. but it is consistent motivation across all of her killings at Camp Arawak and a form of her standing up for herself.

Age also is not factor as Angela herself is quite young (14 at the time of the first film) and was obviously socially and emotionally stunted by the trauma of watching her sister and father die and the negative reinforcement of Aunt Martha. This does not excuse her killing (any of them) but it explains it. If killing kids her own age vs. pre-teens is what makes or breaks the morality of her actions for you then you're really just splitting hairs.

Obviously NONE of what I read this film as was the desired result of it's creators who wrote it as a way of exploited gay panic but that doesn't change the fact that the numbers add up.

P.S. There is a lot we don't know about what happens before the prologue to Sleepaway Camp but John and Lenny are 100% a couple. If Lenny and Martha were married then why is he not there to see his kids off to camp? Why would he, if we entertain your theory, a closeted gay man let Martha condition Peter into a girl like that? If they maybe having a secret fling then why did they have a family beach day together? You are taking a long walk for a very short drink of water when there is clearly a more obvious answer in front of you if you don't read them as a couple.